The present invention relates to an image forming apparatus such as a laser printer and a digital copying machine, an optical beam scanning device used for the image forming apparatus, and a lens. Particularly the invention relates to an overillumination scanning optical system whose width in a main scanning direction of incident light flux into a polygon mirror is broader than a plane width in the main scanning direction of the polygon mirror.
An optical beam scanning device is used in the laser printer apparatus, the digital copying machine, and the like which are of an electrostatic copying type image forming apparatus, in which an electrostatic latent image is formed with a laser beam and a visualized (developer) image is obtained by developing the electrostatic latent image. In the optical beam scanning device, the image (original image) to be output is divided into a first direction and a second direction orthogonal to the first direction, and a light beam whose light intensity is changed is repeatedly output in a substantially linear shape at predetermined time intervals based on image data in either the separated first or second direction, i.e., the light beam is scanned. The image corresponding to the original image is obtained by moving a recording medium or a latent image bearing body at constant speed in the direction orthogonal to the scanned light beam during a time interval between the scannings of the one-line light beam and the subsequent one-line light beam or during the scanning of the one line.
In the optical beam scanning device, the first direction in which the light beam is scanned is usually referred to as main scanning direction. The second direction orthogonal to the first direction is usually referred to as a sub-scanning direction. In the image forming apparatus, the sub-scanning direction corresponds to a transfer material conveying direction, and the main scanning direction corresponds to the direction perpendicular to the conveying direction in a transfer material plane. In the image forming apparatus, an image surface corresponds to the transfer material surface, and an imaging surface corresponds to a surface on which the beam is actually imaged.
In the above image forming apparatus and optical beam scanning device, generally the following relationship holds among image process speed (for example, conveying speed of the recording medium such as paper or the latent image bearing body), image resolution, motor revolving speed, and the number of planes of a polygon mirror:
                              P          ×          R                =                              25.4            ×            Vr            ×            N                    60                                    (        1        )            where    P (mm/s): process speed (sheet conveying speed),    R (dpi): image resolution (the number of dots per inch),    Vr (rpm): the number of revolutions of polygon motor, and    N: the number of planes of polygon mirror.
From the equation (1), it is found that the process speed (namely, print speed) and the image resolution are proportional to the number of planes of the polygon mirror and the number of revolutions of the polygon motor. Therefore, in order to realize speed enhancement and high resolution of the image forming apparatus, it is necessary that the number of planes of the polygon mirror is increased and the number of revolutions of the polygon motor is increased.
In underillumination type (generic term when compared with the overillumination type) optical beam scanning devices which are currently used in many image forming apparatuses, the width (cross-sectional beam diameter, or beam diameter when the main scanning direction differs from the sub-scanning direction in the width) in the main scanning direction of the light beam (light flux) incident to the polygon mirror is limited so as to be smaller than the width in the main scanning direction of an arbitrary reflection plane of the polygon mirror. Accordingly, the light beam guided to each reflection plane of the polygon mirror is entirely reflected by the reflection plane.
On the other hand, the cross-sectional beam diameter (beam diameter in the main scanning direction when the main scanning direction differs from the sub-scanning direction in the diameter) of the light beam guided to the recording medium or the latent image bearing body (image surface) is proportional to an F number Fn of an imaging optical system. At this point, the F number Fn can be expressed by Fn=f/D, where f is a focal distance of the imaging optical system and D is a diameter in the main scanning direction of the light beam in an arbitrary reflection plane of the polygon mirror.
Accordingly, in order to enhance the resolution, when the cross-sectional beam diameter of the light beam is decreased on a scanning subject (image surface), i.e., the recording medium or the latent image bearing body, it is necessary to increase the cross-sectional beam diameter in the main scanning direction in each reflection plane of the polygon mirror. Therefore, when both the plane width of each reflection plane of the polygon mirror and the number of reflection planes are increased, the polygon mirror becomes enlarged. When the large polygon mirror is rotated at high speed, a large motor having a large torque is required, which results in cost increase in the motor, the increases in noise and vibration, and heat generation. Therefore, the countermeasures against these problems are required.
On the contrary, in the overillumination type optical beam scanning device, the width in the main scanning direction of the light beam with which each reflection plane of the polygon mirror is irradiated is set so as to be larger than the width in the main scanning direction of each reflection plane of the polygon mirror, so that the light beam can be reflected by the total plane of each reflection plane. Accordingly, the number of reflection planes of the polygon mirror, the image formation speed, and the image resolution can be increased without increasing the dimension of the polygon mirror, particularly the diameter beyond necessity. Further, in the overillumination type optical beam scanning device, the total diameter of the polygon mirror itself can be decreased, and the number of reflection planes can be increased. Therefore, in the overillumination type optical beam scanning device, a shape of the polygon mirror comes close to a circle and the air resistance is decreased, so that a polygon mirror load is decreased, the noise and the vibration are suppressed, and the heat generation can be suppressed when compared with the underillumination type. Further, since the countermeasure components such as glass required to decrease the noise and vibration can be eliminated or the number of countermeasure components can be decreased, there is also a cost-down effect in the overillumination type optical beam scanning device. Further, a high-duty cycle can be realized. For example, the overillumination scanning optical system is described in Laser Scanning Notebook (Leo Beiser; SPIE OPTICAL ENGINEERING PRESS).
When the optical beam scanning device is formed by one imaging lens (usually referred to as fθ lens), a fluctuation in section thickness is increased. When rotation about a main scanning axis is generated in the case where an imaging-lens fitting error exists, there is a problem that a lens surface fluctuation amount is increased because of a long distance to the lens surface and thereby an optical property of the imaging lens is largely worsened.